The present invention relates to a protection circuit for a detachable operating unit used in an audio device. More specifically, it relates to a protection circuit for a detachable terminal such that when the detachable operating unit is connected to a main audio device, the main audio device applies two different voltages to the detachable operating unit by two power supplies through two contact points, respectively.
In an automotive audio device for an automobile such as a car stereo, a car compact disk player and a car radio, a detachable operating unit 3 having an input key unit 1 and a liquid crystal display unit 2 is detachable from a main audio device 4 as shown in FIG. 2. When the car is stopped or parked, the detachable operating unit 3 may be separated from the main audio device 4 to prevent the audio device from being stolen.
The detachable operating unit 3 comprises an IC circuit 5 controlling the input key unit 1, the liquid crystal display panel unit 3, and an illumination lamp 6 for illuminating a surface of the liquid crystal display unit 2.
An operating voltage VDD of the IC circuit is, for example, +5 V at a relatively low level. On the other hand, an operating voltage ILL of the illumination lamp 6 is, for example, +14 V at a relatively high level. Therefore, both operating voltages could not be supplied from a single power supply unit. As a result, the operating voltages are supplied by two power supply units through power supply terminals 7a and 7c, respectively. A contact terminal 7b is used for the ground (GND).
However, in the case where the detachable operating unit 3 having such a structure for the operating voltages VDD and ILL separately applied to the detachable operating unit 3 is engaged with or disengaged from the main audio device, if the timing of connection of the ground terminal GND to the main audio device is delayed from that of the power supply terminals 7a and 7c, a reverse voltage in response to a differential voltage between the operating voltages ILL and VDD (for example, 5 V-14 V= -9 V) is inadvertently applied to the IC circuit 5 thereby damaging the IC circuit.
Therefore, to overcome the above-described problems inherent in the conventional apparatus having the operating detachable portion, a detection switch 8 has been included on the engaging surface of the main audio device. This detecting switch 8 detects whether the operating detachable unit 3 is engaged or disengaged. As a result, once the switch 8 determines that a operating detachable unit has been engaged with the main audio device without a failure, the operating voltages VDD and ILL are thereafter supplied to the power supply terminals 7a and 7c, respectively. However, to prevent the IC circuit from being broken by controlling the supply timing of the operating voltages to the power supply terminals, the audio device is required to include a detecting switch, an ON/OFF switching unit of the power supply, a control circuit for controlling the ON/OFF switching unit of the power supply, and software for controlling the ON/OFF switching unit and control circuit, so as to increase the number of mechanical and electronic parts and additional processing. The mounting space of audio device is also increased, resulting in an undesired enlargement of the device.